I’m not a lawyer, but I followed a few similar discussions in various opensource projects, so I can try to answer, but be aware that I’m not sure I’m right
Sonarsource left this repository with an outdated copyright, and I try to understand who is now holder of this copyright.
You only have intellectual property of the code you write. The copyright will expire after a very long time, or it can be transferred (like when you sign a CLA).
As I’m now owner of this repository, did I became the new copyright owner too ?
I don’t think that’s enough. You would be better covered by asking explicitly to get copyright transfer. I will ping our legal team on this.
Are this kind of modification as big as expected to allows the licence change
You can change the license if you are the copyright owner. You are by default the copyright owner of the code you write (or the ownership can be transferred).
In theory no modifications are big enough to make you the only author of the code. You would be co-author, and still have to ask permission to all authors to relicense. Or you have to write entire new code.
In practice, if changes are big enough, I think the frontier between modified and new code is blurry enough to allow claiming the ownership, but that certainly the play field of some lawyers.